(bug 36310) jquery.byteLimit should skip < 0 due to Firefox bug.

(bug 36310) jquery.byteLimit should skip < 0 due to Firefox bug.

* Firefox returns -1 if an element has no maxLength property set.
  (instead of undefined). This is especially problematic because
  roundtripping that value (e.g. setting elem.maxLength = -1;)
  causes a DOMException to be thrown.
  That exception is expected (and happens also in Chrome), but Chrome
  and other standard browsers don't return -1 for the getter, so it
  would never end up in the setter.

* Moved the check to before the property set/removal. Otherwise
  the exception would still be thrown. This should've been before
  anyway.

* This fixes:
  - (bug 36310) Can't enter a log summary on Special:MovePage
  in Firefox.

  Even though we do set maxLength for that field from PHP. Recently
  the input field was changed from <input> to <textarea>. And since
  English Wikipedia has wgHtml5 disabled and maxLength on <textarea>
  is new in HTML5, that property was stripped by the Html class, and
  the javascript incorrectly assumed  that the property would be there
  and thus caused the DOMException in Firefox.

* Added unit test for this (which fails in Firefox without this fix).

Change-Id: I28490b61bd5a20dd1d84c4730615514d3822aa77